In the field of medicine, dentistry or the like, pulsed lasers are used for the removal of hard and soft body tissues such as dental enamel, dentine, bone material, skin and mucosa. The material removal in tissue ablation is based on a pronounced absorption of the temporally limited laser pulse in the ablated tissue. The laser absorption leads to local heating with sudden evaporation that causes material removal.
Depending on the laser intensity I (optical power per unit area) two distinct ablation regimes can be encountered in pulsed laser ablation. At low intensities, the speed of ablation is lower than the speed at which the laser-generated heat diffuses away from the ablated surface area deeper into the tissue. In this regime, the thermally influenced layer of tissue that is not ablated by the end of the laser pulse is relatively thick. For most medical procedures this “hot” regime is not desirable, since it may result in thermal tissue necrosis.
At high laser intensities, however, the ablation front progresses into the tissue faster than heat diffusion. In this “cold” regime, most of the preheated tissue is eventually ablated by the end of the pulse, and the amount of remaining heat which is deposited within the tissue is low.
It is to be appreciated that for the same laser pulse energy, long pulses have lower intensity and therefore ablate in the “hot” regime, while short pulses have high intensity and ablate in the clinically desirable “cold” regime.
The sudden material evaporation generated by individual laser pulses also results in a cloud of removed gases, liquids and solid particles (further on referred to also as “debris”, “debris cloud”, “debris particles” or “cloud particles”) above the treated location, wherein the cloud begins to form immediately after the onset of each laser pulse and interacts with the impinging light until the end of the laser pulse. When the individual pulse impinges on the aforementioned debris cloud, the cloud particles get rapidly heated up to very high temperatures, sometimes leading to plasma formation within the surrounding gases and air. As a result, pulsed laser ablation is typically accompanied by undesirable high intensity visible and UV light being emitted from the debris cloud area. Additionally, the rapid absorption of the laser light in the debris cloud enhances the burning smell and popping sound that normally accompany pulsed laser ablation. Further, as the laser-heated debris cloud falls back onto the tissue surface, it contributes to the heating of the tissue. All these accompanying effects are unpleasant and potentially unsafe for the practitioner and the patient.
The size of the debris cloud and hence the above-described undesirable effects can be reduced to a certain degree by delivering laser energy with pulses of longer duration. Here, “size of the debris cloud” means the amount of debris particles which the laser beam has to cross, in order to reach the treated region of the tissue (the amount of debris particles can be measured in mole or gram). With laser pulses of longer duration, both the size of the debris cloud and the intensity of the impinging laser light are lower, and consequently the undesirable effects of the interaction of the laser light with the debris cloud are reduced.
In conclusion, the shorter and more intense the pulses are, the less undesirable heat remains deposited within the treated tissue, after the pulse has ended. Typically, pulses shorter than 350 microseconds are used to achieve ablation in the cold regime. On the other hand, the undesirable effects of the interaction of the laser light with the debris cloud are much less pronounced at longer pulse durations 350 microseconds) and lower pulse intensities.
The invention has the object to develop a laser system of the aforementioned kind which operates at longer pulse durations 350 microseconds) for which the above-described undesirable effects resulting from the interaction of the pulsed laser light with the debris cloud are considerably reduced, without significantly increasing the amount of heat that remains deposited within the treated tissue.